Influenza A(H1N1)pdm09 but not A(H3N2) virus infection induces durable sero-protection: results from the Ha Nam Cohort.

BACKGROUND The extent to which influenza recurrence depends upon waning immunity from prior-infection is undefined. We used antibody titres of Ha-Nam cohort participants to estimate protection curves and decay trajectories. METHODS 270 households participated in influenza-like-illness surveillance and provided blood at intervals spanning RTPCR-confirmed transmission. Sera were tested in haemagglutination inhibition assay. Infection was defined as RTPCR+ influenza-like-illness and/or seroconversion. Median protective titres were estimated using scaled-logistic-regression to model pre-transmission titre against infection status in that season, limiting analysis to households with infection(s). Titres were modelled against month since infection using mixed-effects linear regression to estimate decay and when titres fell below protection-thresholds. RESULTS 295 and 314 participants were infected with H1N1pdm09-like and A/Perth/16/09-like (H3N2Pe09) viruses, respectively between December 2008-2012. The proportion of householders not-infected (protected) rose more steeply with titre for H1N1pdm09 than for H3N2Pe09, and estimated 50% protection titres were 19.6 and 37.3, respectively. Post-infection titres started higher against H3N2Pe09 but decayed more steeply than against H1N1pdm09. Sero-protection was estimated to be sustained against H1N1pdm09 but to wane by 8-months for H3N2Pe09. CONCLUSIONS Estimates indicate that infection induces durable sero-protection against H1N1pdm09 but not H3N2Pe09, which could in part account for the younger age of A(H1N1) versus A(H3N2) cases.